gridview textbox validation

Question

function in javascript: 

function updatealert()

{

var f = document.getElementById('<%=(gvStock.FooterRow.FindControl("txtQnty")).ClientID%>');

if(f.value =='' ")

{

alert("Enter Quantity");

return false;

}

}

gridview template:

 

<asp:Button runat="server" ID="btnUpdate" Text="Update" onkeypress="return validate(event)" OnClick="btnUpdate_Click" OnClientClick="return updatealert()" ForeColor="#003c71"></asp:Button> 

 im getting this error: 

An exception of type 'System.NullReferenceException' occurred in App_Web_ol5f1rt3.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object